Hormones Explain Why Girls Like Dolls & Boys Like Trucks

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When offered the choice of playing with either a doll or a toy
truck, girls will typically pick the doll and boys will opt for
the truck. This isn't just because society encourages girls to be
nurturing and boys to be active, as people once thought. In
experiments, male adolescent monkeys also prefer to play with
wheeled vehicles while the females prefer dolls — and their
societies say nothing on the matter.

The monkey research, conducted with two different species in 2002
and 2008, strongly suggested a biological explanation for
children's toy preferences. In recent years, the question has
become: How and why does biology make males (be they monkey or
human) prefer trucks, and females, dolls?

New and ongoing research suggests babies' exposure to hormones
while they are in the womb causes their toy preferences to emerge
soon after birth. As for why evolution made this so, questions
remain, but the toys may help boys and girls develop the skills
they once needed to fulfill their ancient gender roles.

First, in 2009, Gerianne Alexander, professor of psychology at
Texas A&M University, and her colleagues found that 3- and
4-month-old boys' testosterone levels correlated with how much
more time they spent looking at male-typical
toys such as trucks and balls compared with female-typical
toys such as dolls, as measured by an eye tracker. Their level of
exposure to the hormone androgen during gestation (which can be
estimated by their digit ratio, or the relative lengths of their
index and ring fingers) also correlated with their visual
interest in male-typical toys.

Kim Wallen, a psychologist at Emory University who has studied
the gender-specific toy preferences of young rhesus monkeys,
said, "The striking thing about the looking data shows that the
attraction to these objects occurs very early in life, before
it's likely to have been socialized."

Further buttressing the idea that toy preferences are caused by
hormones, last year, a group of British researchers found that
girls with a condition called congenital adrenal hyperplasia, who
experienced abnormally high levels of the male sex hormone
androgen while in the womb, prefer to play with male-typical
toys. [ Why
Is Pink for Girls and Blue for Boys? ]

But why would male sex hormones make people favor wheeled
vehicles and balls? A common explanation holds that these toys
facilitate more vigorous activity, which boys are evolutionarily
programmed to seek out. But the 2009 study indicated that their
affinity for balls and trucks predates the stage when children
actually start playing with toys. At just 3 months old, the
newborn boys already fixed their eyes on the toys associated with
their gender.

"Given that these babies lack physical abilities that would allow
them to 'play' with these toys as do older children, our finding
suggests that males preference for male-typical toys are not
determined by the activities supported by the toys (i.e.,
movement, rough play)," Alexander said.

Wallen approaches the data more cautiously. "It's hard to
interpret what the looking data mean because we don't know why
people are attracted to specific things. Clearly children
recognize that certain objects in their environment are
appropriate for certain activities. They could be looking at a
certain toy because it facilitates an activity they like," he
said.

The debate over why boys prefer toy vehicles and balls continues.
In a new study, Alexander and her colleagues investigated whether
19-month-olds move around when playing with trucks and balls more
than they do when playing with dolls. According to the study,
they don't. Toddlers with higher levels of testosterone are more
active than toddlers with lower levels of the sex hormone, but
the active toddlers moved around just as much when holding a toy
truck, ball or doll. "We find no evidence to support the widely
held belief that boys prefer toys that support higher levels of
activity," she wrote in an email. A paper detailing the work has
been accepted for publication in the journal Hormones and
Behavior.

If it isn't vigorous activity they're after, it could be that
boys simply find balls and wheeled vehicles more interesting,
while human figures appeal more to girls. As for why evolution
would program these toy preferences, the researchers have a few
ideas. According to Alexander, one possibility is that girls have
evolved to perceive social stimuli, such as people, as very
important, while the perceived worth of social stimuli (and thus,
dolls that look like people) is weaker in boys. [ The
Smarter Sex? Women's Average IQ Overtakes Men's ]

Boys, meanwhile, tend to develop superior spatial navigation
abilities. "Multiple studies in
humans and primates shows there is a substantial male
advantage in mental rotation, which is taking an object and
rotating it in the mind," Wallen said. "It could be that
manipulating objects like balls and wheels in space is one way
this mental rotation gets more fully developed."

This is purely speculative, Wallen said, but boys' superior
spatial abilities have been tied to their traditional role as
hunters. "The general theory is that well-developed skills in
mental rotation allowed long distance navigation: using an
egocentric system where essentially you navigate using your
perception of your location in 3D space," he said. "This might
have facilitated long distance hunting parties."